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United States Patent |
6,080,352
|
Dunfield
,   et al.
|
June 27, 2000
|
Method of magnetizing a ring-shaped magnet
Abstract
A brushless permanent-magnet direct current motor has a permanent magnet
which is magnetized to create an offset angle between detent and mutual
torques for providing sufficient starting torque for all relative
orientations between the stator and the rotor of the motor. This is
accomplished by providing a permanent magnet in which the global
magnetization of the magnet has been disrupted by the application of a
local magnetic field to a portion of the magnet, thereby to provide a
magnetic anomaly in the global magnetization. Also, a method of
magnetization of the magnet is described.
Inventors:
|
Dunfield; John C. (Chatsworth, CA);
Heine; Gunter K. (Aptos, CA);
Jufer; Marcel (Morges, CH);
Oveyssi; Kamran (Aptos, CA)
|
Assignee:
|
Seagate Technologies, Inc. (Scotts Valley, CA)
|
Appl. No.:
|
828487 |
Filed:
|
March 31, 1997 |
Current U.S. Class: |
264/427; 29/607; 148/103; 148/108; 264/DIG.58; 425/3 |
Intern'l Class: |
B29C 071/04; H01F 013/00 |
Field of Search: |
264/427,DIG. 58
29/607
148/103,108
425/3
|
References Cited
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4935655 | Jun., 1990 | Ebner | 310/154.
|
4948999 | Aug., 1990 | Bertram et al. | 310/162.
|
4968913 | Nov., 1990 | Sakamoto | 310/156.
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4992710 | Feb., 1991 | Cassat | 331/825.
|
5028852 | Jul., 1991 | Dunfield | 318/254.
|
5093599 | Mar., 1992 | Horng | 310/254.
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5117155 | May., 1992 | Buhler | 315/8.
|
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|
5191269 | Mar., 1993 | Carbolante | 318/254.
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5191270 | Mar., 1993 | McCormack | 318/254.
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5221881 | Jun., 1993 | Cameron | 318/254.
|
5233275 | Aug., 1993 | Danino | 318/254.
|
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|
5254914 | Oct., 1993 | Dunfield et al. | 318/254.
|
5294856 | Mar., 1994 | Horst | 310/181.
|
5343127 | Aug., 1994 | Maiocchi | 318/254.
|
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|
5783890 | Jul., 1998 | Mulgrave | 310/156.
|
Foreign Patent Documents |
772324 | Oct., 1934 | FR.
| |
1033643 | Jul., 1953 | FR.
| |
1 518 886 | Jul., 1978 | GB.
| |
Primary Examiner: Silbauch; Jan H.
Assistant Examiner: Poe; Michael
Attorney, Agent or Firm: LaRiviere, Grubman & Payne, LLP
Parent Case Text
RELATED APPLICATION DATA
This application is a continuation-in-part of Ser. No. 08/479,619 filed
Jun. 7, 1995, now abandoned, which is a divisional of Ser. No. 08/273,535
filed Jul. 11, 1994, now abandoned.
Claims
What is claimed is:
1. A method of magnetizing a ring shaped magnet to provide an offset angle
between detent and mutual torques of a single-phase, brushless, direct
current spindle motor for a hard disc drive, the method comprising the
steps of:
applying a global magnetic field to the magnet thereby to apply a generally
uniform global magnetization to the magnet;
applying a local magnetic field to a segment of the magnet to disrupt the
global magnetization of the magnet in that segment of the magnet, wherein
resulting magnetic disruptions introduce detent and mutual torque phase
shifts that provide a net phase shift, thereby providing the offset angle
between detent and mutual torques.
2. A method according to claim 1 wherein the strength of the local magnetic
field is between 5% and 20% of the strength of the global magnetic field.
3. A method according to claim 2 wherein the global magnetization of the
magnet is disrupted by the application of the local magnetization over
less than 30.degree. of the magnet at each location where the local
magnetization is applied.
4. A method according to claim 1 wherein the steps of applying the global
and local magnetic fields comprises the steps of:
locating the magnet proximate to a first magnetizing coil;
energizing the first magnetic coil to create a global magnetic field
through the magnet which is sufficient to create a generally uniform
global magnetization of the magnet;
locating the magnet proximate to a second magnetizing coil;
energizing the second coil to create a local magnetic field in a segment of
the magnet, thereby to disrupt the global magnetization of the magnet in
the segment of the magnet.
5. A method according to claim 4 wherein the locating steps take place
simultaneously by means of the positioning of the magnet between the first
and second coils.
6. A method according to claim 4 wherein the strength of the local magnetic
field is between 5 and 20% of the strength of the global magnetic field.
7. A method according to claim 4 wherein the magnet is located between 1
and 5 mils from the second magnetizing coil.
8. A method according to claim 4 wherein the global magnetization of the
magnet is disrupted by the application of the local magnetization over
less than 30.degree. of the magnet at each location where the local
magnetization is applied.
9. A method according to claim 8 wherein the local magnetic field is
applied to the magnet at two diametrically opposed locations on the
magnet.
10. A method according to claim 9 wherein the two diametrically opposed
locations at which the local magnetic field is applied are offset from an
axis of symmetry of the global magnetization by approximately 15.degree..
Description
TECHNICAL FIELD
The present invention generally relates to permanent-magnet direct current
(PMDC) electric motors, and more specifically relates to internal stator,
brushless PMDC single-phase spindle motors for use in hard disc drives.
BACKGROUND ART
PMDC motors employ magnetized materials to supply pole flux. Some materials
employed for use in PMDC motors may be found among several classes of
permanent-magnet materials, including alnicos, neodymium-iron-borons,
ceramics (ferrites), and rare-earth materials. Because a permanent-magnet
(PM) is used, the flux per pole, .PHI., is not adjustable in PMDC motors.
The static torque which can be applied to an unexcited motor without
causing continuous rotation is termed the detent torque. The latter torque
arises in PM motors owing to the continual presence of a magnetic field
even in the absence of applied current (excitation) to the armature
winding.
The position at which a PM rotor comes to rest without excitation at
no-load is termed the detent position. The detent position is the location
where detent torque equals zero.
In PMDC motors, the armature circuit includes, in part, an armature winding
about a portion of the stator core. The armature winding is sometimes
referred to as a stator coil or stator winding. In brushless PMDC motors,
the armature winding receives a current which produces a "revolving" flux
in the air gap between the stator and rotor. This revolving flux causes
the PM rotor to revolve with respect to the stator for producing torque.
The torque produced when the motor is excited is a combination of mutual
and detent torques. Mutual torque arises from the forces of attraction and
repulsion between the poles of the PM and the opposite electromagnet poles
of the excited stator coil.
As is known, motors may be single-phase or polyphase. In single-phase PMDC
motors, only one set of armature windings is employed. Thus, one or more
"coils" in series may be used for the armature winding, while retaining
the single-phase characteristic.
A problem associated with single-phase PM motors is starting. Single-phase
PM motors detent at detent torque nulls (locations where no detent torque
exists). If these detent torque nulls coincide with mutual torque nulls,
then there is no torque for starting the motor when current is applied. In
order to start a single-phase PMDC motor, current must be delivered to the
motor, and the PM rotor must be at a position where useful torque is
delivered to the motor. As applied to spindle ("spinning") motors for hard
disc drives, single-phase PMDC motors must be started prior to obtaining
constant rotation.
DISCLOSURE OF INVENTION
In accordance with the present invention, a single-phase PMDC motor
provides an offset angle between null detent and null mutual torques to
provide sufficient starting torque for starting the motor. Because the
rotor PM is not located at a torque null at start-up, the present
invention ensures that the motor is in a starting position when excited.
Hence, the motor may be started by the interaction of flux (e.g., from a
PM) and coil excitation. By economically providing an offset angle between
mutual and detent torques, the present invention facilitates manufacture
of a low cost single-phase motor with performance characteristics
sufficient for spindle motor hard disc drive operation.
The present invention thus introduces an offset angle between detent torque
and mutual torques to avoid locating the rotor PM at a zero starting
torque location. Prior to exciting the motor, the rotor PM is at a detent
position. The PM poles of the rotor are thus located at a detent torque
null. When the stator coil is excited, the PM poles will tend to align
with the electromagnetic poles of the stator, i.e., a mutual torque null
location. As the detent and mutual torque null locations are offset from
one and other, this translates into physical movement (rotation) of the PM
pole for alignment with the electromagnetic poles of the stator. (This of
course assumes sufficient current is supplied to the stator coil for
overcoming the detent torque.) In other words, owing to introduction of an
offset angle, there will be mutual torque at the detent torque null
location. Therefore, the motor will be capable of delivering torque to the
motor for causing the rotor to rotate when the stator coil is excited.
In accordance with the present invention, an offset angle may be introduced
by shifting both the detent and mutual torque. While phase shift of the
detent torque may not equal phase shift of the mutual torque, the combined
shift of detent and mutual torques provides a net shift, namely, an offset
angle. Moreover, in accordance with the present invention, the proportion
of phase shift of the mutual torque may be significantly larger than any
phase shift of the detent torque.
There are several different mechanisms for introducing the offset angle in
an internal stator, PMDC, single-phase motor in accordance with the
present invention.
In one embodiment, a variable reluctance is introduced by pole notches
formed in internal stator shoes. Owing to the variable reluctance and
asymmetric magnetic field distribution, the rotor PM will detent off axis,
i.e., a phase shift in detent torque. Also, mutual torque will be phase
shifted owing to reorientation of the magnetic poles of the stator by
"removal" of mass for forming pole notches. Mutual torque is shifted in
phase an amount which is different from the shift in phase of detent
torque such that mutual and detent torque nulls do not coincide.
In another embodiment, slots are made in an internal stator. The slots are
narrow, thus variable reluctance and magnetic field distribution effects
are small, i.e., small phase shift, if any, in detent torque. However, the
slots create saturation regions in the stator shoes, such that flux
distribution is asymmetrical, i.e., significant phase shift of mutual
torque.
In another embodiment, complete portions of the stator shoes are omitted.
Thus, magnetic field distribution about the stator tooth is asymmetrical.
This causes phase shifts in both detent and mutual torques.
In another embodiment, four slots are provided in the stator. In one of the
four slot embodiments, additional starting poles are symmetrically
disposed about the stator tooth; and in another four slot embodiment,
additional starting poles are asymmetrically disposed about the stator
tooth. In both of these embodiments, the PM is radially magnetized causing
magnetic flux to flow in the starting poles. Consequently, mutual torque
is phase shifted. In the asymmetrically disposed starting poles
embodiment, both detent and mutual torques are phase shifted.
A starter coil may be added onto the additional starting poles of the four
slot embodiments. The starter coil may be used for starting a single-phase
PMDC motor independent of any shift from detent torque nulls. In other
words, no detent torque is needed to start the motor, and consequently the
motor may be started when positioned at a detent torque null location. The
starter coil may also be used for sensing back electromotive force (emf)
nulls, i.e., "zero crossings."
Using, the starter coil, the motor of the present invention may be brought
up to speed by open loop control, e.g., with an appropriate timed ramp for
drive pulses, or a closed loop control, e.g., sensing back electromotive
force (emf).
In other embodiments the rotor PM is magnetized with magnetic anomalies.
The magnetic anomalies introduce both mutual and detent torque phase
shifts.
In other embodiments, the stator is made in two pieces to optimized the
distance between pole pieces. Moreover, winding the stator coil is
simplified for manufacturing purposes.
In other embodiments, the stator is external. The external stator
embodiment may also include an internal stator. The motor magnetic circuit
(a combination of a coil and a section of the stator core) of the external
stator embodiment may be extended to a printed circuit board (PCB) for
direct coupling to motor drive electronics. The coil may be wound onto a
section of the stator, which in turn is attached to a PCB.
The stator may be made of solid steel, laminated steel, laminated sheets of
steel insulated from one another, and like media. However, other media
suitable for conducting magnetic flux may be used. Also, the molecules
comprising the magnetic media may be oriented for improved flux carrying
capacity and detent torque characteristics. Molecular orientation of
magnetic materials is known in the art, and thus a discussion of same is
omitted in order to avoid confusion while enabling those skilled in the
art to practice the claimed invention.
Other features of the present invention are disclosed or apparent in the
section entitled "BEST MODE FOR CARRYING OUT THE INVENTION."
BRIEF DESCRIPTION OF DRAWINGS
For fuller understanding of the present invention, reference is made to the
accompanying drawing in the following detailed description of the Best
Mode of Carrying Out the Invention. In the drawing:
FIG. 1 is a graphical representation of starting and detent torques where
an offset angle has been introduced between the two torques;
FIG. 2A is a side view in cross-section of a prior art hard disc drive;
FIG. 2B is an enlargement of a portion of the hard disc drive of FIG. 2A;
FIG. 3A is a top exposed and partial cross-section view of a portion of a
single-phase motor in accordance with the present invention;
FIG. 3B is a top exposed and partial cross-section view of a portion of an
alternate embodiment of a single-phase motor in accordance with the
present invention;
FIG. 4 is a top exposed and partial cross-sectional view of a portion of a
two slot version of a single-phase motor in accordance with the present
invention;
FIG. 4A shows magnetic flux lines based on a finite element analysis for an
diametrically magnetized magnet for the motor of FIG. 4;
FIG. 5 is a top exposed and partial cross-sectional view of an alternate
embodiment of a portion of a two slot version of a single-phase motor in
accordance with the present invention;
FIG. 5A shows magnetic flux lines based on a finite element analysis for an
radially magnetized magnet for the motor of FIG. 5;
FIG. 6 is a top exposed and partial cross-sectional view of a portion of a
four slot version of a single-phase motor in accordance with the present
invention;
FIG. 7 is a top, exposed view of an alternate embodiment of a four slot
version of a portion of a single-phase motor in accordance with the
present invention;
FIG. 8 is a top exposed and partial cross-sectional view of an alternate
embodiment of a portion of a two slot version of a single-phase motor in
accordance with the present invention having magnetic anomalies;
FIG. 9 is a top exposed and partial cross-sectional view of the alternate
embodiment of the motor of FIG. 8 with different magnetic anomalies;
FIG. 10 is a top, partial cross-sectional view of an external stator
alternate embodiment in accordance with the present invention;
FIG. 11 is a top, partial cross-sectional view of another external stator
alternate embodiment in accordance with the present invention;
FIG. 12 shows magnetic flux lines based on a finite element analysis for a
portion of the stator of FIG. 11 having an diametrically magnetized
magnet;
FIG. 13 is a cross-section of an external stator test motor in accordance
with the present invention;
FIG. 14 is a circuit diagram in accordance with the present invention for
starting a motor and sensing back emf in accordance with the present
invention;
FIG. 15 is a top exposed and partial cross-sectional view of an alternate
embodiment of a portion of a two piece stator single-phase motor in
accordance with the present invention;
FIGS. 16 and 17 are cross-sectional views of alternate embodiments of the
motor of FIG. 15 in accordance with the present invention;
FIG. 18 is a top plan view of a stator coil body of the motor of FIG. 15 in
accordance with the present invention;
FIG. 19 is a top plan view of a stator pole piece of the motor of FIG. 15
in accordance with the present invention;
FIG. 20 is a graphical representation of back emf voltage and encoder
voltage;
FIG. 21 is a plot of current rise versus time for a brushless PMDC motor;
FIG. 22 is a circuit diagram for open loop control in accordance with the
present invention;
FIG. 23 are graphic illustrations of two examples of signals having
different starting locations;
FIG. 24 is an enlargement of FIG. 8 showing the positioning of a
magnetization coil for creating the magnetic anomalies;
FIG. 25 is a plan view of a magnetizer for creating the magnetic anomalies
in the permanent magnet illustrated in FIGS. 8 and 24; and
FIG. 26 is a schematic representation of an electrical circuit for the
magnetizer illustrated in FIG. 25.
Reference numbers refer to the same or equivalent parts of the present
invention throughout the several figures of the drawing.
BEST MODE FOR CARRYING OUT THE PRESENT INVENTION
Introduction
Referring to FIG. 1, there is shown a graphical representation of mutual
torque voltage 7 and detent torque voltage 6 plotted as y-coordinate
voltage, axis 1, versus x-coordinate voltage, axis 2. It should be
understood that mutual torque voltage is directly proportional to current
delivered to a motor, whereas detent torque is inherent, i.e., not
affected by application of current. As shown, an offset angle 8 (phase
shift) exists between starting torque voltage 7 and detent torque voltage
6. Thus, detent torque null 4 and starting torque null 3 do not coincide.
Thus, for all orientations of a PM rotor there will exist starting torque,
whether mutual torque, detent torque, or a combination of both, capable of
being delivered the single-phase PMDC motor in accordance with the present
invention.
As the present invention relates to single-phase, brushless PMDC spindle
motors for use in hard disc drives, a more detailed understanding of the
background art of spindle motors is appropriate.
Background
In FIG. 2A, a cross-section of a prior art hard disc drive 10 is shown, of
which an enlarged view of circled region 11 is shown in FIG. 2B.
Referring to FIG. 2A, hard disc drive 10 includes base casting 13, driver
board 12, and spindle motor 14. Spindle motor 14 is located in drive
volume 15 and attached to base casting 13.
Now referring to FIG. 2B, permanent magnet 25 is connected to rotor 22, and
stator coil 23 is wrapped around stator core 24 and connected to drive
circuits (not shown). Torque for spinning rotor 22 and magnet 25 is
created by applying current to coil 23 to induce magnetic flux. The magnet
flux between rotor magnet and stator coil poles cause the rotor to spin.
Depending on the initial-starting orientation of rotor magnet and stator
coil poles, the rotor may spin clockwise (CW) or counter-clockwise (CCW)
upon motor start-up.
In spindle motor applications, it is critical that the rotor spin the same
direction each time for writing or reading information to or from one or
more discs, e.g., disc 20. Therefore, the magnetic orientation of poles of
the rotor and stator must be known from the start. In brushless motors,
indirect means such as sensors or detectors are used for determining the
magnetic orientation of the poles, i.e., determining the position of
magnetic poles of magnet 25 attached to rotor 22. By employing such
sensors or detectors, it is possible to energize the stator coil in such a
manner that the rotor spins the same direction every time. Other means for
detecting the position of the rotor for causing the motor to spin the same
direction each time are described in U.S. Pat. No. 5,028,852, issued to
John C. Dunfield, one of the listed inventors, and incorporated by
reference as though fully set forth herein.
Before proceeding with a detailed description of the present invention as
applied to a spindle motor, it should be understood that the principles of
the present invention may be adapted for use with a variety of different
systems other than hard disc drives.
Two Pole Notch Embodiments
Referring to FIG. 3A, there is shown a top exposed and partial
cross-sectional view of a portion of a motor 30 constructed in accordance
with the principles of the present invention. Although portions of rotor
39 and coil 37 are not shown in FIG. 3, it should be understood that:
rotor 39 covers magnet 31, stator 36, and coil 37 in a similar fashion as
does rotor 22 of FIG. 2B; and coil 37 is wrapped around stator 36,
avoiding rotor shaft or bearing opening 35, essentially forming a parallel
electromagnetic circuit. Opening 35 is optional, and depends upon
configuration of motor 30. It should be understood that in this type of
internal stator configuration, magnet 31 is attached to rotor 39 and
revolves around the stator during operation of the motor. Air gap 34 is
created between stator 36 and magnet 31 and is the space between the outer
diameter of stator 36 and the inner diameter of magnet 31. It should
further be understood that magnet 31 may be replaced with more than one
magnet.
Two openings 32 are formed in stator 36, as well as two pole notches 33.
While two pole notches 33 are illustrated, it should be understood that
one or more pole notches 33 may be employed in accordance with the present
invention. However, in a one pole notch 33 configuration (not shown),
there would be torque asymmetry about 180 degrees, and consequently such a
configuration may not be desirable.
Openings 32 and pole notches 33 form air gaps in stator 36. Reluctance
depends in part on the geometry of the air gaps. In stator 36, openings 32
increase the reluctance. Also, the reluctance is increased at pole notches
33 as compared to other locations along air gap 34.
Owing to the asymmetry introduced by pole notches 33, magnet 31 will detent
asymmetrically about the tooth of stator 36, as indicated by north, N, and
south, S, poles of magnet 31 being distributed on either side of central
stator axis 9. Thus, rotor 39 will detent such that magnetic mass of
stator 36 is approximately evenly distributed, i.e., poles of magnet 31
will align with magnetic mass center line 3. Because magnet 31 detents off
axis 9, detent torque null locations of motor 30 do not coincide with
mutual torque null locations. Consequently, an offset angle is introduced
between mutual and detent torques. The amount of this offset angle is
proportional to the amount of asymmetry introduced by pole notches 33. The
more magnetic mass "removed" from stator 36 to provide pole notches 33,
the greater the offset angle introduced.
Additionally, magnet 31 may be magnetized to enhance the amount of offset
angle. For example, magnet 31 may be radially or diametrically magnetized.
The effect of radial magnetization versus diametrically magnetization will
depend on stator 36 geometry, as the angle at which magnetic flux of
magnet 31 impinges the contour of stator 36 will effect the offset angle.
For radial magnetization, it is preferable to use an anisotropic
magnetically permeable material, and for diametrically magnetization, it
is preferable to use an isotropic magnetically permeable material.
Radially magnetized magnets include S32H from TDK and SAM17R from Seiko
Epson. Diametrically magnetized magnets include Neom11 from TDK or Diado.
Referring to FIG. 3B with continuing reference to FIG. 3A, there is shown a
top exposed and partial cross-sectional view of a portion of a motor 300
constructed in accordance with the principles of the present invention.
Motor 300 is similar to motor 30, except that stator 306 is shaped
differently than stator 36. Also, opening 35 has been omitted. Because
stator 306 is of a different shape that stator 36, slots 32 and air gaps
34 are different than those of motor 30. However, the principles of
operation as described with respect to motor 30 also apply to motor 300.
Saturation Slot Embodiment
Now referring to FIG. 4, there is shown a top exposed and partial
cross-sectional view of a portion of an alternate embodiment of a motor 40
constructed in accordance with the principle of the present invention.
Again, portions of rotor 39 and coil 37 have been omitted. However, it
should be understood that coil 37 is wrapped around the tooth of stator
46, avoiding opening 35.
Asymmetrical air gaps are created between stator 46 and magnet 31 by slots
43 and 42. Air gaps are also created by the difference between the outer
diameter of stator 40 and the inner diameter of magnet 31, e.g., air gaps
44. Slots 43 cut through laminations of stator 46 alter the flux paths.
While this "removal" of mass from stator 46 may provide some off axis 9
detent, owing to the limited amount of mass removed and the narrowness of
slot 43 there will be little impact on shifting the detent position of
magnet 31. However, the change to mutual torque will be significant. Slots
43 create localized saturation regions 48 in stator 46 (when excited).
Regions 48 are narrow, thus creating a magnetic flux "bottleneck" when
stator 46 is excited. Consequently, not all the flux is able to travel
through region 48 for even distribution about the stator shoes. Some flux
is thus diverted away from the slotted regions 47 of the stator shoes
toward the non-slotted regions 45 of the stator shoes. As the magnetic
flux cannot evenly distribute, it effectively appears as an asymmetrical
distribution of magnetic field of stator 46. Thus, creating a offset angle
between detent and mutual torques sufficient for providing starting torque
for any orientation of magnet 31.
Again, while two slots 43 are illustrated, it should be understood that one
or more slots 43 may be employed in accordance with the present invention.
However, in a one slot 43 configuration (not shown), there would be torque
asymmetry about 180 degrees, and consequently such a configuration may not
be desirable.
Referring to FIG. 4A, a diagram 40A shows the results of a finite element
analysis for the motor 40 of FIG. 4, indicating flux lines 49 of a
magnetic field for stator 46, rotor 39 and magnet 31 at a rest position.
Flux lines 49 along magnet 31 indicate diametrical magnetization in FIG.
4A, but the same principles would apply if magnet 31 had been radially
magnetized.
Flux lines 49 indicate localized saturation regions 48 corresponding to
slots 43 of FIG. 4. While shaping of flux lines 49 is shown for the top of
stator 46, this shaping equally applies to the bottom of stator 46.
Asymmetrical Shoe Embodiment
In FIG. 5, there is shown a top exposed and partial cross-sectional view of
a portion of an alternate embodiment of a motor 50 constructed in
accordance with the principles of the present invention. Again, portions
of rotor 39 and coil 37 have been omitted. However, it should be
understood that coil 37 is wrapped around the tooth of stator 56, avoiding
opening 35. Air gaps are created between stator 56 and magnet 31 by slots
52 in stator 56, as well as the difference between the outer diameter of
stator 56 and the inner diameter of magnet 31, e.g., air gaps 54.
In motor 50, stator shoes 58 (or pole arcs) are asymmetrical about stator
tooth 57, and the magnetic field distribution is asymmetrical about axis
9. This results in a offset angle between mutual and detent torques
sufficient for providing starting torque at any position of magnet 31.
Referring to FIG. 5A, a diagram 50A shows the results of a finite element
analysis of the motor 50 of FIG. 5, indicating lines of flux of a magnetic
field for stator 56, rotor 39 and magnet 31 at a rest or start position.
Flux lines 55 in FIG. 5A indicate that magnet 31 is diametrically
magnetized, but the same principles would apply if magnet 31 had been
radially magnetized. While this shaping of the lines of flux is shown for
the top of stator 56, the same shaping of the flux occurs at the bottom of
stator 56. Notably, diametrical magnetization of magnet 31 will enhance
facilitation of the offset angle.
Four Slot Embodiments
In FIGS. 6 and 7, there are shown four-slot versions of a single-phase
motor in accordance with the present invention.
Referring to FIG. 6, there is shown a top exposed and partial
cross-sectional view of a portion of an alternate embodiment of a motor 60
in accordance with the present invention. Again, a portion of rotor 39 has
been omitted. Also, a portion of coil 67 has been omitted. However, it
should be understood that coil 67 is wrapped around the tooth of stator
66, avoiding opening 35.
Air gaps are created between stator 66 and magnet 31 by four slots 62 in
stator 66, as well as the difference between the outer diameter of stator
66 and the inner diameter of magnet 31, e.g., air gaps 64.
Magnet 31 is radially magnetized as indicated by arrows 45. While mass
distribution will be symmetrical about axis 9, radially magnetic flux
flows to poles 68, as well as additional starter poles 69, of motor 60.
When current is applied to coil 67 of motor 60, a four pole
electromagnetic stator is created. As poles 69 do not have corresponding
opposite poles to align with (i.e., poles 68 are aligned to magnetic poles
of magnet 31), poles 69, when electromagnetized, will tend to align with
poles of magnet 31. Thus, a starting torque for motor 60 is present.
Start on Mutual Torque Only No Detent Torque, Embodiment
In another approach to starting motor 60, starter winding 65 is fitted
around poles 69 of stator 66. In this embodiment an additional phase is
added to poles 69 of motor 60. As starter winding 65 is only energized
momentarily it may comprise a large number of turns of a fine wire to
produce a starting torque, as well as suitable voltage for sensing.
Starter winding 65 has sufficiently large number of turns for providing
sufficient starting torque to overcome internal motor frictions, as well
as move the rotor. As starter winding 65 will have a high resistance and a
high back emf constant, a starting torque can be achieved with the
application of only a small amount of current to winding 65. Moreover, as
starter winding 65 can supply a starting torque, either winding 65 or coil
67 may be excited for detenting motor 60. In other words, coil 67 may be
excited in order to position magnet 31 for starting motor 60 with winding
65. Or, winding 65 may be excited in order to position magnet 31 for
starting motor 60 by exciting coil 67.
The method of starting motor 60 with winding 65 may first encompass
attempting to start motor 60 prior to applying current to winding 65. If
motor 60 failed to start (i.e., rotor 39 is at a zero detent torque
position), winding 65 may be excited to start motor 60. Because phase of
winding 65 is at quadrature (shifted .pi./4 radians or 90 electrical
degrees) with phase of coil 67, when coil 67 is at minimum torque, winding
65 is at maximum torque. Alternatively, both coil 67 and winding 65 may be
energized for starting motor 60. Motor 60 may be run off coil 67 once
started, and winding 65 may then be de-energized.
Therefore, it should be understood that employing winding 65 differs
completely from using detent torque to start a single-phase PMDC motor. No
detent torque is needed to start motor 60, rather motor 60 may be started
off mutual torque only. Mutual torque nulls are avoided by using either
winding 65 or coil 67 to start motor 60.
Back EMF Sensing Using Starter Coil
Normally, winding 65 would over heat owing to its high resistance; however,
because winding 65 is only current driven for a short time this is not a
problem. Therefore, winding 65 may include a large number of turns of a
fine wire (or suitably dimensioned wire for the working volume).
The number of turns may be such that the back emf voltage induced across
winding 65 would ultimately exceed the power supply voltage, if winding 65
were excited up to full speed. Yet because winding 65 is only energized
for a short period of time, the rotor velocity is not too high and the
back emf voltage never reaches a point at which motor 60 could not be
operated. The flux change with respect to time for the slight rotation to
start motor 60 does not produce back emf voltage sufficient to overcome
the power supply.
However, because winding 65 is provided with a large number of turns, the
back emf voltage induced across its many turns supplies sufficient voltage
for sensing back emf without any current supplied to winding 65 from a
power supply, while coil 67 is driven.
A circuit analog for driving motor 60 is shown in FIG. 14. As shown in FIG.
14, an H-bridge integrated circuit (IC) 142 is implement. The H-bridge IC
142 may be an L298 SGS. Coil 67 is coupled to an H-bridge of IC H-bridge
142 as well as is winding 65. Winding 65 is also coupled in parallel to
comparator 140. The output of comparator 140 is coupled to timing circuit
141, whose output is coupled to H-bridge IC 142. This implementation
allows full two-phase operation and is only one implementation of a
circuit in accordance with the present invention. Alternatively, switching
circuits other than an H-bridge may be employed in accordance with the
principles of the present invention. For example, winding 65 may be
energized, with current being switched on and off through a single
transistor, for producing starting torque in a single-phase manner.
Commutation of a single-phase motor occurs after back emf crossings, i.e.,
zero crossings. Thus, the current supplied to the motor must be turned off
prior to a back emf crossing and turned back on after the back emf
crossing. Winding 65 may be used for sensing these zero crossings.
As magnet 31 (shown in FIG. 6) is rotated, current is induced in winding
65. the current will have a positive or negative direction depending on
the passing polarity of magnet 31 (shown in FIG. 6). Thus, voltage across
winding 65 will change from positive to negative. The voltage across
winding 65 is the input voltage to comparator 140. As comparator 140 is
configured as a zero crossing detector, each time the voltage across
winding 65 changes, comparator 140 output will change or toggle. Each
change in sign of voltage across winding 65 corresponds to a back emf zero
crossing location.
Timing circuit 141 may be implemented to turn current on and off, through
an H-bridge of IC 142, to coil 67 with the toggling of the output of
comparator 140, allowing for quadrature shift. Thus, winding 65 may be
used to sense back emf crossings for turning current on and off as applied
to coil 67.
Referring now to FIG. 20, there are shown graphical representations of back
emf voltage 201 and encoder voltage 202. Back emf voltage 201 is shown
plotted on voltage axis 200 versus time axis 205, as is encoder voltage
202. Back emf voltage 201 represents the induced voltage across starter
winding 65 (shown in FIG. 6), when starter winding 65 (shown in FIG. 6) is
not supplied current by the power supply. Encoder voltage 202 represents
an output of comparator 140 (shown in FIG. 14) corresponding to induced
back emf voltage 201. Because back emf voltage 201 indicates zero
crossings 203, encoder voltage produces commutation "crossings" 204.
As explained above, it is necessary to determine what polarity of current
must be initially applied to the motor for causing the desired direction
of rotation, e.g., positive torque. If the desired direction of rotation
is not achieved, the polarity of current supplied to the motor may be
reversed to produce positive torque.
Referring now to FIG. 21, there is shown a graphical representation of
current rise verses time for a single-phase, brushless, PMDC motor.
Referring to FIGS. 6 and 7 with continuing reference to FIG. 21, either
winding 65 or coil 67, 77 may be used for sensing rise time difference.
For example, in a single-phase application, winding 65 is supplied with a
positive current followed by a negative current. Using application of
differing polarities of current, the proper current direction may be
determined for producing positive torque by defining a fixed current
threshold 207 along current axis 206. Using threshold 207, it is possible
to measure current rise times for each application of current to winding
65. Curve 208 represents application of positive polarity current to
starter winding 65. Curve 209 represents application of negative polarity
current (absolute value thereof) to starter winding 65. Time 211 is the
point at which threshold current 207 is reached along positive current
curve 208. Time 212 is the point at which threshold current 207 is reached
along negative current curve 209. The sign of time difference 213 is
characteristic of the direction of rotation of motor 60 or 70. Thus, time
difference 213 may be used to set polarity of current supplied to motor 60
or 70 to cause positive torque (i.e., where emf and current polarities are
the same).
A processing means, such as a microprocessor or other suitable processor,
may be employed in timing circuit 141 (shown in FIG. 14) for determining
time difference 213. Also, timing circuit 141 (shown in FIG. 14) may
include any of a variety of well known memory means, such as random
access, read only, flash, or like memory, to record times 211, 212. Again,
referring to FIG. 14, it should be understood that commutation is
controlled in a closed loop manner. However, the present invention may
also commutate with open loop control.
Referring to FIG. 22, there is shown a circuit analog for driving a motor
under open loop control. In circuit 220, sensor voltage (as represented by
sensor resistor 365) appears at node 221. Resistor 365 is in series with
coil 337 (e.g., coil 67 shown in FIG. 6) being excited for receiving
applied current. Thus, current supplied to coil 337 will produce a voltage
at node 221. Voltage at node 221 is input to comparator 140 for comparison
with reference voltage appearing at node 223 for toggling output of
comparator 140. Sensing voltage at node 221 for comparison with a
reference voltage at node 223 allows back emf zero crossings to be
determined for purposes of commutation.
Again referring to FIG. 21, sign of time difference 213 indicates direction
of rotation of rotor magnet (e.g., magnet 31 of motor 60 shown in FIG. 6).
Moreover, magnitude of time difference 213 may be used to determine the
location of a rotor magnet. In other words, the magnitude of time
difference 213 may be used to determine where, in the positions between
zero crossings, e.g., commutation crossings 204 (shown in FIG. 20), a
magnet (e.g., magnet 31 shown in FIG. 6) is located. Thus, the time
interval between the next commutation crossing and between subsequent
commutation crossings may be determined such that current may be supplied
to a motor (e.g., motor 60 shown in FIG. 6) in the form of a "timed" ramp
drive pulse.
Referring to FIG. 23, there is shown a graphical representation of an
output of comparator 140 of circuit 220 (shown in FIG. 22). Two cases are
shown, namely, case 230 and case 231. In case 230, a motor (e.g., motor 60
shown in FIG. 6) has a starting location 232, resulting in time difference
234. In case 231, a motor (e.g., motor 60 shown in FIG. 6) has a starting
location 233, resulting in time difference 235. Time difference 235 is
larger than time difference 234, as signal 230 must travel farther than
signal 237 for encountering a first commutation crossing 204. At
approximately location 238, normal acceleration takes hold. Thus, a timed
ramp drive pulse may be based upon total torque (T) per second moment of
inertia (J), i.e., acceleration (T/J).
Referring again to FIG. 22, therefore, timing circuit 141 pulses ramp
circuit 225 for supplying current to coil 67 through H-bridge circuit 142.
The duration of the time between pulses from timing circuit 141 depends on
the location of magnet 31 (shown in FIG. 6).
Asymmetrical Four Slot Embodiment
Now referring to FIG. 7, there is shown a top exposed and partial
cross-sectional view of a portion of an alternate embodiment of a motor 70
in accordance with the present invention. Again, a portion of rotor 39 has
been omitted. Also, a portion of coil 77 has been omitted. However, it
should be understood that coil 77 is wrapped around the tooth of stator
76, avoiding opening 35. Air gaps are created between stator 76 and magnet
31 by four slots 72 in stator 76, as well as the difference between the
outer diameter of stator 76 and the inner diameter of magnet 31, e.g., air
gaps 74.
Again, magnet 31 is radially magnetized. The same principles apply to motor
70, as those described with reference to motor 60 of FIG. 6 with one
difference. Poles 79 are asymmetrically disposed about the tooth of stator
76. Consequently, the asymmetrically disposed starter poles 79, when
electromagnetized, will tend to align more readily with the poles of
magnet 31, as compared with poles 69 of motor 60 of FIG. 6 which are
equidistant from the poles of magnet 31 at the start position. Also, poles
79 asymmetrically distribute magnetic mass about axis 9 of stator 76.
Optionally, motor 70 may be fitted with starter winding 65 around poles 79
of stator 76. Starter winding 65 operates in similar fashion as described
herein with reference to motor 60 of FIG. 6.
Magnetic Anomalies Embodiments
FIGS. 8 and 9 show alternate embodiments of motors in accordance with the
present invention having magnetic "anomalies" introduced into the
magnetization pattern of a permanent magnet. The anomalies are for
providing a shift between detent torque and mutual torque.
Referring to FIG. 8, there is shown a top exposed and partial
cross-sectional view of a portion of an alternate embodiment of a motor 80
in accordance with the present invention. Again, a portion of rotor 39 has
been omitted. Also, a portion of coil 37 has been omitted. However, it
should be understood that coil 37 is wrapped around the tooth of stator
86, avoiding opening 35.
Air gaps are created between stator 86 and magnet 31 by slots 42 in stator
86, as well as the difference between the outer diameter of stator 86 and
the inner diameter of magnet 31, namely, air gaps 84.
While magnet 31 is radially magnetized to provide radial magnet flux, as
indicated by arrows 81, magnet 31 may alternatively be diametrically
magnetized in this embodiment. Magnet 31 also includes magnetic anomalies,
as indicated by curved lines 88. These magnetic anomalies cause magnet 31
to align off axis 9. Consequently, a offset angle is introduced between
mutual and detent torque of motor 80 such that torque nulls do not
coincide.
Now referring to FIG. 9, there is shown a top exposed and partial
cross-sectional view of a portion of an alternate embodiment of a motor 90
in accordance with the present invention. Again, portions of rotor 39 and
coil 37 have been omitted. However, it should be understood that coil 37
is wrapped around the tooth of stator 86, avoiding opening 35.
Air gaps are created between stator 86 and magnet 31 by slots 42 in stator
86, as well as the difference between the outer diameter of stator 86 and
the inner diameter of magnet 31, namely, air gaps 84.
Magnet 31 is radially magnetized, as indicated by arrows 91. Radial
magnetization of magnet 31 is disposed about two opposing arc length
sections along its circumference. Magnet 31 is also diametrically
magnetized, as indicated by arrows 92, and such magnetization forms two
opposing arc length sections along the circumference of magnet 31. In the
embodiment shown, the radial magnetization sweeps through two 45 degree
arcs, and the diametrical magnetization sweeps through two 135 degree
arcs. However, the present invention is not limited to arcs of 45 and 135
degrees, and a variety of other angles may be used. In accordance with the
present invention, anomalies may be introduced into a magnet using both
radial and diametrical magnetizations.
Combining differing magnetizations in magnet 31 with stator 86 having
laminations results in a saturation-induced magnetic field shaping effect.
The saturation-induced magnetic field causes magnetic flux to be
distributed unevenly. The magnetic anomalies (radial and diametrical
magnetization) cause magnet 31 to align off axis 9, thus, introducing a
offset angle to avoid coincidence of mutual and detent torque nulls.
While the embodiments of FIGS. 8 and 9 have been described with two
anomalies each, it should be understood that the present invention may be
practiced with fewer or more than two anomalies in magnet 31. Thus, one or
more flux disturbances may be employed for providing a shift between
detent and mutual torque.
Two Piece Stator Embodiments
Referring now to FIG. 15, there is shown a top exposed and partial
cross-sectional view of a portion of an alternate embodiment of a motor
150 in accordance with the present invention. As indicated by arrows 151,
magnet 31 is diametrically magnetized. The stator of motor 150 is made up
of two components, namely, stator pole piece 152 and stator coil body 153
or 157. One manner of introducing an offset angle between mutual torque
and detent torque is to provide notches around the stator.
An introduced offset angle may be optimized by to reducing the distance
between the pole pieces to zero. Consequently, motor 150 is built in two
pieces, namely stator pole piece 152 and stator coil body 153 or 157.
Motor 150 further includes coils 154 wound about bobbin 155. Coil 154 is
wound onto stator coil body 153 or 157.
Referring to FIG. 19, there is shown a top plan view of stator pole piece
152. As shown, stator pole piece 152 includes slot (air gap) 168 and
notches 156. Slot 168 creates two saturation regions 158 in stator pole
piece 152. Saturation regions 158 become saturated with magnetic flux when
motor 150 (shown in FIG. 15) is excited. The thin magnetic bridge forming
saturation regions 158 should be in the range of lamination thickness, so
that regions 158 are saturated approximately equivalent to air gap 159
shown in FIG. 15. Saturation regions 158 in combination with notches 156
introduce a offset angle between detent and mutual torques in order to
avoid coincidence of torque nulls, such that there is starting torque for
all positions which rotor 39 and magnet 31 (shown in FIG. 15) may be
oriented.
Referring to FIG. 18, there is shown a top plan view of stator coil body
153, 157. As shown, coil 154 is wound around bobbin 155. Bobbin 155 is
disposed on stator coil body 153 or 157 for this purpose. Stator coil body
153 or 157 also comprises poles 158. While poles 158 have arcuate stator
shoe shapes for providing an expanded area for routing flux from stator
coil body 153 or 157 to stator pole piece 152; however, it should be
understood that stator coil body 153 or 157 need not be so shaped. In
fact, stator coil body 153 or 157 may be shaped in any suitable manner
allowing for it to be attached under stator pole piece 152 and support
coil 154. Stator coil body 153 or 157 may be fixed under stator pole piece
152 by adhesive, glue, epoxy, soldering, or any other suitable known
method of attachment. Because the stator of motor 150 (shown in FIG. 15)
is made from two pieces, stator coil body 153 or 157 may have coil 154
wound thereon prior to attachment to stator pole piece 152. This allows
for much simpler manufacturing as the wire comprising coil 154 does not
have to be threaded through ends of slot 168 as shown in FIG. 15.
Now referring to FIG. 16 in conjunction with FIG. 17, there are shown
cross-sectional views of motor 150. In FIG. 16 stator pole piece 152 is
shown as being made up of two laminations 161, 162. Additionally, stator
coil body 153 is shown as being made up of two laminations 163, 164. In
FIG. 17, stator coil body 157 is shown having a curved profile made up of
two laminations 166, 167. By curving stator coil body 157, coil 154 may be
shifted further away from rotor 139 as compared with coil 154 wound on
stator coil body 153 shown in FIG. 16.
Owing to flux variation, eddy current losses appear. To reduce eddy current
losses, laminations may be added to form stator pole piece 152 (shown in
FIG. 19). Additionally, radial slots 143 (shown in FIG. 19) may optionally
be added to stator pole piece 152 (shown in FIG. 19) to further facilitate
introduction of an offset angle as was described with respect to slots 43
of FIG. 4.
While a spindle motor having a rotating shaft, with an internal stator
configuration has been described, other configurations of motors may be
used when practicing the present invention. For example some other
configurations include a stationary shaft motor, an in-hub motor, and the
like.
External Stator Embodiments
Another motor in accordance with the present invention is of an external
stator configuration for providing a large space for a rotor bearing and
for a yoke (for mounting the stator winding). The external stator
configuration also allows an external stator winding to be located on a
PCB, where the PCB may include the circuitry for operating the motor.
Optionally, an internal stator may be employed for additional flux
shaping. Use of an internal stator allows flux shaping to be enhanced
without exceeding allocated space for the motor.
Referring to FIG. 10, there is shown a top, partial cross-sectional view of
a portion of an alternate embodiment of a motor 100 constructed in
accordance with the principles of the present invention. A cross section
of rotor backiron 104 is shown. Rotor backiron 104 is a portion of the
hubshaft of the rotor. Optionally, rotor backiron 104 may define an
opening 350 for receipt of an internal stator therein. Coil 107 is wrapped
around stator 106. Stator 106 has four lobes 108, two notches 195 and a
quasi-rectangular contour 109. Notches 195 create saturation regions 194
such that flux distributes through stator 106 as indicated by arrows 193.
Contour 109 in combination with two lobes 108 and a notch 195 define an air
gap 102. Air gaps are also created between circular-like contour 199 of
stator 106 and magnet 101 by the difference between an inner diameter of
stator 106 and an outer diameter of magnet 101, namely, air gap 119. Air
gap 119 is larger at pole notches 103 located along contour 199 of stator
106. Pole notches 103 are for shifting the detent torque from the mutual
torque, as explained above. For further facilitating the shift between
detent and mutual torque, magnet 101 may be directionally magnetized.
Magnet 101 may be radially magnetized, diametrically magnetized, radially
and diametrically magnetized, radially or diametrically magnetized with
one or more anomalous magnetized regions, and the like.
Thus, it should be understood that above-described offset means with
respect to internal stator motors may be employed with respect to external
stator motors as well.
Lobes 108 shape flux for reducing detent torque. As this shaping reduces
the detent torque, less current is needed to start motor 100. Lobes 108
also reduce magnetic flux leakage by increasing the amount of iron around
magnet 101. Lobes also provide increased mutual torque for motor 100.
Moreover, detent torque and mutual torque are substantially constant for
motor 100 with lobes 108, zero crossing regions excepted.
The core of stator 106 is made as one piece. However, the core of stator
106 may be made of more than one piece. For example, stator 106 may be
made in three pieces, e.g., two separate pole sections 178 and a third
section 179 for the coil. All three pieces may be joined for good flux
conduction.
Constructing stator 106 from three pieces, optionally allows winding the
coil on the third section, and then attaching the wound coil and third
section onto PCB 118. The pole sections may then be attached for forming
stator 106.
Stator 106 may also be made as a solid or laminated structure. The
structure may be made of any of a variety of known materials, including a
steel alloy composition having silicon and the like.
Now referring to FIG. 11, there is shown a top, partial cross-sectional
view of a portion of a motor 110 constructed in accordance with the
principles of the present invention. While not illustrated, it should be
understood that coil 117 is wrapped around stator 116. Stator 116 has four
lobes 108, two notches 195, a circular-like contour 199 and a
quasi-semicircular contour 119. Contour 119 in combination with a notch
195 and two lobes 108 define air gap 112. Quasi-semicircular contour 119
is formed to correspond with lobes 108 to minimize air gap 112 for
improved power delivery to motor 110 from an excited coil 117 with fewer
turns on coil 117 as compared with coil 107 of motor 100 shown in FIG. 10.
Stator 116 is made as one piece. However, stator 116 may be made of more
than one piece. For example, stator 116 may be made in three pieces, e.g.,
two separate pole sections and a third section for the coil. All three
pieces may be joined for good flux conduction.
Constructing stator 116 from three pieces, optionally allows winding of the
coil on the third section, and then attaching the wound coil and third
section onto PCB 118 (shown in FIG. 10). The pole sections may then be
attached for forming stator 116. Stator 116 may also be made as a solid or
laminated structure.
Optionally, a second internal stator 197 may be added to motor 110. Stator
197 may also be added to motor 100 (not shown with an internal stator).
Stator 197 includes a slot 196 for forming an air gap therein. Outer
diameter of stator 197 and inner diameter of backiron 104 define air gap
198. Stator 197 can be employed to further shape the detent torque of
motor 110, especially in situations involving limited available space.
Referring to FIG. 12, a diagram 120 of the results of a finite element
analysis of motor 110 of FIG. 11 is shown indicating lines of magnetic
flux. Flux lines 129 indicate that magnet 101 is diametrically magnetized,
but the same principles would apply if magnet 101 had been radially
magnetized. While this shaping of the lines of flux is shown for the top
of stator 116, the same equally applies to the bottom of stator 116.
Referring to FIG. 13, there is shown a cross-section of an external stator
configuration motor 130. Motor 130 is shown mounted to a test mounting
plate 121; however, motor 130 may be mounted to a hard disc drive base
casting, PCB, or like platform. Motor 130 is made up of stator 136, coil
137, rotor hubshaft 139, magnet 131, bearings 122, sleeve 124, and
bearings 123.
Working Model
A working model of an interior stator, rotating shaft, brushless PMDC
spindle motor 30 of FIG. 3B was built in accordance with the principles of
the present invention. The parameters are given in Table A.
TABLE A
______________________________________
Magnet outer diameter 18.38 mm
Magnet inner diameter 16.38 mm
Air gap .125 mm
Stack height 1.8 mm
Magnet height 1.8 mm
Hub thickness 1.2
______________________________________
mm
Notably, these parameters are suitable for spindle motors.
The results from the working model are given in Table B.
TABLE B
______________________________________
Magnetic detent torque .74 mNm
Mutual torque at .390 A
2.27 mNm
______________________________________
The motor was operated by installing an optical detector aligned for
detecting back emf crossings driving through a bi-polar H-bridge. The
H-bridge was implemented with an L298 SGS integrated circuit. The sign of
the motor current to produce starting torque in the appropriate direction
was hard wired to avoid starting difficulties. Using this configuration,
the detent torque and electromagnetic torque zeros differed by
approximately 10 degrees, and the motor started at approximately 0.24
amps.
FIG. 24 is an enlarged view of the magnetic anomaly embodiment of the
invention illustrated in FIG. 8. Accordingly, the earlier description of
the FIG. 8 embodiment also applies to FIG. 24, and thus the entire earlier
description will not be repeated here, for purposes of conciseness.
FIG. 24 illustrates a top exposed and partial cross-sectional view of a
portion of the motor 80. The illustrated portion of the motor 80 comprises
a magnet 31, a coil 37, and a stator 86. The rotor 39 has been omitted
from FIG. 24. As before, the magnet 31 has a global radial magnetization,
as indicated by arrows 81, but it may alternatively be diametrically
magnetized. The magnet 31 also includes magnetic anomalies, or disruptions
in the global magnetization, as indicated by curved flux lines 88. These
disruptions in the global magnetization cause an offset angle to be
introduced between the mutual and detent torques of the motor 80 such that
torque nulls do not coincide.
Also shown in FIG. 24 is a single wire magnetization coil 350, which is
used to generate the magnetic anomaly flux lines 88. The wires of the
single wire magnetization coil 350 are substantially straight as they pass
by the magnet 31. Accordingly, the magnetic field imposed on the magnet 31
when the magnetization coil 350 is energized is substantially the magnetic
field generated when current flows through a single straight conductor,
i.e. the flux lines are a series of concentric circles centered about the
conductor. This can be seen from the shape of the flux lines 88
illustrated in FIGS. 8 and 24.
The field generated by the single wire magnetization coil 350 is
approximately one tenth of the magnetic field imposed on the magnet 31 in
the course of its overall global magnetization, as will be discussed in
more detail below. Accordingly, the magnetizing effect of the single wire
magnetization coil only has a local effect on the magnet 31, which serves
to disrupt the otherwise generally uniform global magnetization. In this
regard, it should be noted that the disruption or anomaly is created
deliberately in the global magnetization, and the disruptions are thus
different from transitions between areas of radial and diametral
magnetization, or transitions between the direction of magnetization.
Also, it will be appreciated the term "generally uniform magnetization"
includes the transition areas between areas of radial and diametral
magnetization, or transitions between the direction of magnetization,
which arise inherently in a permanent magnet having areas of differing
magnetization or magnetization directional changes.
In the illustrated embodiment, each magnetic anomaly or disruption occupies
an arc of less than 30.degree., extending from the axis of symmetry or
North/South axis 352 of the overall magnetization. The single wire
magnetization coils are therefor offset by approximately 15.degree. from
the axis 352 during application of the local magnetization.
It will of course be appreciated that the single wire magnetization coils
350 are not actually present in the motor 80 as shown, but are rather
present in a magnetizing device as described below with reference to FIG.
25.
A magnetizing device, generally indicated by the numeral 360, for
magnetizing the magnet 31 with a radial magnetization and two disruptions
or magnetic anomalies is illustrated in FIG. 25. The magnetizing device
comprises a first ring 362 made of a steel with a high magnetic saturation
value, a second smaller ring 364 made of the same material, and first and
second single wire magnetization coils 366, 368. The first and second
magnetization coils 366, 368 are located in notches defined in the first
and second rings 362, 364 respectively as shown.
The magnetizing device 360 is constructed in accordance with the general
principles of magnetizer design, which are well known in the magnetizer
art and will thus not be discussed further here. These general principles
will determine, for example, the selection of the magnetizer backiron, and
the shaping of the steel components.
A gap 370 is defined between the first and second rings 362, 366 for
receiving a ring of hard magnetic material to be magnetized to form the
magnet 31.
The first magnetizing coil 366 is used to generate the radial global
magnetization indicated by the arrows 81 in FIG. 24, and the magnetizing
field is generated by a 100 .mu.s or less pulse of current of 20 kA
through the first magnetizing coil 366. The clearance between the edge of
the first magnetizing coil 366 and the edge of a magnet 31 placed between
the first and second rings 362, 364 is approximately 10 mil (0.254 mm).
The first magnetizing coil is made of #12 wire. Upon excitation, the first
magnetizing coil generates a magnetic field across the gap 370 which is
approximately radial. For purposes of illustration only, a single magnetic
flux line 372 generated by the first magnetizing coil 366 is shown. It
will of course be appreciated that the magnetizing device 360 could be
configured to provide a different global magnetization, such as a
diametral global magnetization, or a combination of diametral and radial
magnetizations.
The second magnetizing coil 368 is used to disrupt the global magnetization
and thus to generate the two disruptions or magnetic anomalies indicated
by the magnetic flux lines 88 in FIG. 24. The local magnetizing field is
generated by a 100 .mu.s or less pulse of current of 2 kA through the
second magnetizing coil 368. The clearance between the edge of the second
magnetizing coil 368 and the edge of a magnet 31 placed between the first
and second rings 362, 364 is approximately 1 mil (0.0254 mm). The second
magnetizing coil is made of #22 wire.
Thus it will be appreciated that the second magnetizing coil 368 is
positioned approximately ten times closer to the magnet 31 than the first
magnetizing coil 366, and that the current passing through the second
magnetizing coil 368 is approximately one tenth of the current passing
through the first magnetizing coil 366. These relationships may be
adjusted within limits to provide a satisfactory disruption of the global
magnetic field. In particular, the current through the second magnetizing
coil 368 is preferably between about 5% and about 20% of the current
through the first magnetizer coil 366, and the second magnetizer coil is
preferably between 0 and 5 mils (0 to 0.127 mm) from the edge of the
magnet 31 in use. Also, the number of turns of the coils 366, 368 may be
varied to provide the desired magnetic field strengths. In general
however, the magnetic field generated by the second magnetizing coil 368
is between 5 and 20% of the magnetic field generated by the first magnetic
coil 366.
A circuit 380 for energizing the first and second magnetizing coils 366,
368 is shown in FIG. 26. The circuit comprises an electrical power source
382, a diode 384, a capacitor 386 and two switches 388, 390. The switches
388 and 390 are operated sequentially to energize the first and second
magnetizing coils 36 and 368. The diode 384 and the capacitor 386 are to
protect the power source 382 from back emf generated when either one of
the switches 388 or 390 are opened.
In use of the magnetizing device 360, the magnet 31 is inserted into the
gap 370. (It will be appreciated here that the "magnet" 31 is not
magnetized yet, and the term "magnet" is used here to include a ring of
the appropriate size and hard magnetic material, which is about to become
a magnet). The first magnetizing coil 366 is energized with a current of
magnitude 20 kA. This applies a global magnetization to the magnet 31, in
this case a radial global magnetization. The first magnetic coil 366 is
then de-energized.
After the magnetic field generated by the first magnetizing coil has
subsided, the second magnetizing coil 368 is energized with a current of 2
kA. This generates a local magnetizing field of approximately one tenth of
the energy of the global magnetizing field. The local magnetizing field as
it passes through the magnet 31 is substantially a segment of the magnetic
field generated when current flows through a single straight conductor,
i.e. the flux lines, as they pass through the magnet 31, are a series of
concentric circular arcs centered about the conductor. This can be seen
from the shape of the flux lines 88 illustrated in FIGS. 8 and 24.
The resulting disruptions of the global magnetization of the magnet 31 each
occupy an arc of less than 30.degree. extending from the axis of symmetry
or North/South axis of the overall magnetization. These disruptions or
magnetic anomalies shift both the detent and the mutual torques of the
motor 80.
It will be appreciated that the disruption of the global magnetic field may
be accomplished in other ways without departing from the spirit and the
scope of teh invention. For example, the above two magnetization steps
could be accomplished in two separate magnetizers.
Other mechanisms for causing a shift between detent and mutual torque may
be employed with variations in geometries in construction of the stator
and/or various other manners of orienting magnetic direction to provide
directionally magnetized magnets. The number of alternate geometries and
magnetization are too numerous to detail within this specification, yet
such alternate approaches are considered within the scope of the present
invention.
The present invention has been particularly shown and described with
respect to certain preferred embodiments and features thereof. However, it
should be readily apparent to those of ordinary skill in the art that
various changes and modifications in form and detail may be made without
departing from the spirit and scope of the inventions as set forth in the
appended claims. The inventions illustratively disclosed herein may be
practiced without any element which is not specifically disclosed herein.
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